The present invention relates to apparatus and a method for removing mercury from the products of solid fuel combustion including flue gases and more particularly to apparatus and a method for removing elemental mercury or mercury compounds from the flue gases from coal combustion.
The use of activated carbon for the adsorption of mercury vapor has been successfully demonstrated in various applications such as municipal waste incineration. However, there are significant differences in the concentration of mercury from waste incinerators compared to coal-fired power plants with the concentration from the coal-fired power plants being anywhere from 10 to 100 times lower. Also, the mercury from waste incinerators is usually in the form of mercury chloride whereas the mercury from coal-fired power plants is usually in the form of elemental mercury. Both of these differences make it more difficult to remove the mercury from the flue gas from a coal-fired power plant.
The utilization factor for activated carbon is limited by the relatively large particle size and low surface area which limits the adsorption of mercury. Using activated carbon with mean particle size of about 5 microns with a top size of about 10 microns would improve the mercury capture efficiency, but storage, handling, transport and dispersion of these articles is extremely difficult. As a result, the use of activated carbon for mercury capture in coal-fired power plants is too costly. In such applications, the utilization of the activated carbon is quite low with a minimum mole ratio of carbon to mercury of 10,000 to 1.
Another form of carbon which has been proposed for the capture of mercury from flue gases is carbon black. Carbon black is a finely divided form of carbon produced by the incomplete combustion or thermal decomposition of a hydrocarbon fuel. The most common form of carbon black is referred to as furnace black or soot which is made by burning natural gas or petroleum oil in a closed furnace with about 50% of the air required for complete combustion. The external surface area of the carbon black is about 100 times that of activated carbon. This could result in a significant decrease of the C/Hg mole ratio for effective mercury capture compared to activated carbon. As the market price for carbon black is similar to that for activated carbon, there is the potential for a significant cost reduction.
Carbon black generation for the capture of mercury from a refuse incinerator is disclosed in the International Patent Application PCT/SE93/00163 (International Publication Number WO 93/20926). This is characterized by the burning of a fuel with a deficiency of oxygen in a separate incinerator and injecting the soot-laden flue gas into the flue gas from the refuse incinerator. However, oxygen deficient combustion leads to the generation of other pollutants such as carbon monoxide and unburned hydrocarbons. Even if the flue gas from the carbon black generator were to be injected upstream of an acid gas removal device such as a flue gas scrubber, the carbon monoxide and unburned hydrocarbons would not be destroyed or removed.
Another problem with the application of prior art carbon black and activated carbon-based methods for mercury capture from incinerators to the capture of mercury from coal-fired power plants is that refuse incinerators have high chlorine levels and the mercury is present in the flue gas predominantly as mercury chloride as previously stated. In coal-fired power plants, the mercury is usually elemental. Although carbon black and activated carbon have a relatively high affinity for the adsorption of mercuric chloride, they have a relatively lower affinity for the adsorption of elemental mercury.
Carbon-based sorbents such as activated carbon have been proposed for controlling vapor phase mercury emissions in power plant flue gases. In a conventional method, carbon sorbents are injected in the flue gas duct upstream of particulate removal device such as baghouses and electrostatic precipitators and downstream of air heaters.
The temperature of the location at which carbon sorbents are injected has heretofore been taken into consideration for the reason that it has been suggested that the adsorption capacity of mercury on carbon sorbents is higher at relatively lower temperatures. In this connection, it is also known in the art that physical adsorption (physisorption) of mercury on carbon is reduced with temperature. Consequently, the prior art essentially does not provide any detailed information concerning the desirability of injecting, for the purpose of capturing mercury from gas streams, carbonaceous sorbents at relatively higher temperatures such as, for example, temperatures above about 400° F.
It is also known in the art that vapor phase mercury in the flue gas emerging from the high temperature boiler is in the form of elemental mercury. Oxidation of elemental mercury to oxidized mercury (Hg2+) is beneficial to mercury control since it can be removed more easily by carbonaceous material.
The injection of activated carbon into the flue gas may typically lead to a presence of, or an increase in, carbon in the fly ash, whereupon an amount of carbon in the fly ash above a prescribed limit may prevent the use of this fly ash for the purpose, for example, of concrete manufacturing. The activated carbon, because of its hydrophobic nature, adsorbs air-entraining additives that are used in the concrete formulation. Also due to activated carbon, the fly ash changes its color to dark grey to black. One way of minimizing this air entrainment impact is to oxidize the carbon, making it more hydrophilic. Another way to make the carbon hydrophilic is by impregnating the carbonaceous sorbent with additives that are hydrophilic, e.g., halide salts such as iron chloride. Still another way of minimizing this problem is to reduce the usage of carbon sorbent below a level that impacts air entrainment behavior and discoloration.